Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that they must be controlled in order to meet government emissions regulations. Among the regulated compounds are nitrogen oxide compounds (NO.sub.x), hydrocarbons, and carbon monoxide. There are a wide variety of combustion processes producing these emissions, for instance, coal- or oil-fired furnaces, reciprocating internal combustion engines (including gasoline and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of NO.sub.x, hydrocarbons, and carbon monoxide are needed.
Industry has devoted considerable effort to reducing regulated emissions from the exhaust streams of combustion processes. In particular, it is now usual in the industry to place a catalytic converter in the exhaust system of gasoline engines to remove undesirable emissions from the exhaust by chemical treatment. Typically, a "three-way" catalyst system of platinum, palladium, and rhodium metals dispersed on an oxide support is used to oxidize carbon monoxide and hydrocarbons to water and carbon dioxide and to reduce nitrogen oxides to nitrogen. The catalyst system is applied to a ceramic substrate such as beads, pellets, or a monolith. When used, beads are usually porous, ceramic spheres having the catalyst metal impregnated in an outer shell. The beads or pellets are of a suitable size and number in the catalytic converter in order to place an aggregate surface area in contact with the exhaust stream that is sufficient to treat the compounds of interest. When a monolith is used, it is usually a cordierite honeycomb monolith and may be precoated with .gamma.-alumina and other specialty oxide materials to provide a durable, high surface area support phase for catalyst deposition. The honeycomb shape, used with the parallel channels running in the direction of the flow of the exhaust stream, both increases the surface area exposed to the exhaust stream and allows the exhaust stream to pass through the catalytic converter without creating undue back pressure that would interfere with operation of the engine.
When a gasoline engine is operating under stoichiometric conditions or nearly stoichiometric conditions with respect to the fuel:air ratio (Oust enough oxygen to completely combust the fuel, or perhaps up to 0.3% excess oxygen), a "three-way" catalyst has proven satisfactory for reducing emissions. Unburned fuel (hydrocarbons) and oxygen are consumed in the catalytic converter, and the relatively small amount of excess oxygen does not interfere with the intended operation of the conventional catalyst system. The stoichiometric conditions or nearly stoichiometric conditions will be referred to as non-oxidizing conditions or as producing a non-oxidizing atmosphere.
However, it is desirable to operate the engine at times under lean burn conditions, with excess air, in order to improve fuel economy. While conventional non-oxidizing engine conditions might have a fuel:air ratio having 0.1-0.3% excess oxygen, a lean burn engine has a substantially greater excess of oxygen, from about 1% to perhaps up to 10% excess oxygen relative to the amount of fuel. Under lean burn conditions, conventional catalytic devices are not very effective for treating the NO.sub.x in the resulting oxygen-rich exhaust stream. Lean burn conditions will be referred to as oxidizing conditions or as producing an oxidizing atmosphere.
The exhaust stream from a diesel engine also has a substantial oxygen content, from perhaps about 2-18% oxygen. It is also believed that other combustion processes result in emissions of NO.sub.x, hydrocarbons, and carbon monoxide that are difficult or expensive to control because of an oxidizing effluent stream or poor conversion of the compounds using conventional means.
In spite of efforts over the last decade to develop a catalytic converter effective for reducing NO.sub.x to nitrogen under oxidizing conditions in a gasoline engine or in a diesel engine, the need for improved conversion effectiveness has remained unsatisfied. The materials developed prior to the present invention have exhibited unacceptably low efficiencies for reduction of NO.sub.x in an oxidizing exhaust stream, even with such high levels of expensive noble metal catalysts as to make them impractical for use by the automotive industry. Moreover, there is a continuing need for improved effectiveness in treating NO.sub.x, hydrocarbons, and carbon monoxide emissions from any combustion process.
The industry has also been concerned with the related problem of the temperatures at which catalytic converter devices are effective for reducing nitrogen oxides and other emissions. Typically, NO.sub.x reduction catalysts are evaluated by the maximum NO.sub.x conversion of the catalyst and the temperature at which that maximum occurs. Automotive exhaust catalysts are expected to perform over a wide range of operating temperatures encompassing cold start (i.e., start when the engine is at ambient temperature) to wide-open throttle conditions. For this reason, a catalyst having a higher peak NO.sub.x reduction performance occurring at one certain temperature may not decrease NO.sub.x emissions as much during the whole period of engine operation as a catalyst having a lower peak NO.sub.x reduction performance but having a wider temperature window over which it has high NO.sub.x reduction activity. Another consideration is the temperature required for a particular catalyst to have any appreciable activity. The standard "three-way" catalyst system is ineffective for treating emissions until a temperature of approximately 250.degree. C., the light-off temperature of the catalyst system, is reached. This threshold temperature for effective operation of the catalytic converter is often referred to as the "light-off" temperature. It would be desirable to reduce the light-off temperature as much as possible because significant amounts of emissions are produced from the time when the engine is started until the catalytic converter is finally heated to the light-off temperature. In addition, diesel engines and engines that are run under lean burn (oxidizing) conditions have lower average exhaust temperatures, usually in the range of about 150 to 350.degree. C. The conventional three-way catalytic converter systems reach maximum efficiency at temperatures between 400 and 800.degree. C., above the operating temperature ranges of these engines.
We have discovered that a coating of an activated noble metal-doped hydrous metal oxide material on a substrate unexpectedly improves the conversion of nitrogen oxides to nitrogen under oxidizing conditions. The coated substrates of the invention also exhibit reduced light-off temperatures over conventional catalytic converters and can be synthesized to provide maximum effectiveness in the operating temperature ranges of diesel engines and of lean-burn gasoline engines.